The present invention relates to a process for producing a material, e.g., a mixture of a carbon black and polytetrafluoroethylene (hereinafter referred to as xe2x80x9cPTFExe2x80x9d) or a mixture of silver and PTFE, for use forming a reaction layer or gas feed layer of a gas diffusion electrode. Specifically, the invention relates to a process for producing the above-described mixture while minimizing the use of a surfactant.
In a conventional process for producing a reaction layer of a gas diffusion electrode, a reaction layer material or a gas feed layer material has been obtained in the following manner. First, a hydrophilic carbon black is dispersed into water containing a surfactant, and a finely particulate catalyst, e.g., silver, is deposited on or added to the dispersed carbon black. Thereafter, a hydrophobic carbon black is dispersed into the dispersion, and a PTFE dispersion is then mixed therewith to prepare a dispersion.
Subsequently, an alcohol is added to the resultant dispersion to self-organize it, and the self-organized dispersion is filtered and dried to obtain a reaction layer material powder or gas feed layer material powder.
In preparing the carbon black dispersion in the conventional process described above, water is added to the carbon blacks in an amount as large as at least 30 times by weight the amount of the carbon blacks and a surfactant is added in an amount at least 2%. After this dispersion is mixed with a PTFE dispersion, an alcohol is added thereto in the same amount as the amount of the resultant dispersion to cause coagulation, and the resultant mixture is filtered and dried to produce the reaction layer material powder or gas feed layer material powder. For inexpensively producing the material powder, it is necessary to reduce the amounts of the water, surfactant, and alcohol used as sub-raw materials.
When the use amounts of these ingredients are reduced, this results in a decrease in the amount of all the raw materials to be treated. As a result, a smaller equipment size is usable and a cost reduction is possible accordingly. Furthermore, in the case where the dispersion containing carbon blacks and PTFE particles is used by any of a coating method in which the dispersion is applied as it is to form a film, a spraying method in which the dispersion is applied by spraying to form a film and a spray drying method in which the dispersion is spray-dried to form a powder, this technique has had a drawback that a large amount of energy is necessary for removing the solution because of too large an amount thereof. It has hence been desired to diminish to the lowest possible level the water and surfactant to be used. In particular, there has been a strong desire for a reduction of the surfactant amount because use of a large amount of a surfactant raises difficulties in removing the surfactant in a later step and in discarding the surfactant as a waste.
The invention has been achieved in view of the above-described problems in the conventional techniques.
Accordingly, an object of the invention is to provide a process for producing gas diffusion electrode materials which is effective in minimizing the use amounts of water and a surfactant and attaining a cost reduction brought about by equipment size reduction, contributes to energy saving, and is effective in shortening the production time.
Other objects and effects of the present invention will become apparent from the following description.
The conventional process requires a surfactant for dispersing carbon blacks into water. In order to overcome this problem, the present inventor made extensive investigations on means for mixing which are sufficient to disperse raw materials into solvents such as, e.g., alcohol solvents, hydrocarbon solvents, and ester solvents, without using a surfactant. Even a hydrophobic carbon black which is not wetted by water is satisfactorily wetted by those organic solvents. It was found that when a PTFE dispersion is added to and mixed with a dispersion prepared by wetting a carbon black by a water-miscible organic solvent, e.g., an alcohol, and dispersing the carbon black thereinto, then the alcohol diffuses into the water that serves as the dispersion medium of the PTFE dispersion and, simultaneously therewith, particles of the carbon black come into spaces among the fine PTFE particles to thereby form a dispersion structure desirable for gas diffusion electrodes. Namely, it was found that when a PTFE dispersion is mixed with a carbon black dispersed in an alcohol, then the PTFE dispersion self-organizes to form a satisfactory reaction layer structure. The invention has thus been completed.
It was further found that a dispersion structure desirable for gas diffusion electrodes is obtained also by wetting a carbon black by an alcohol and dispersing the carbon black thereinto, adding a fine PTFE powder to the dispersion to allow the PTFE particles to sufficiently absorb the solvent, and then subjecting the resultant mixture to an ultrasonic dispersion treatment.
Namely, it has been found that when a carbon black dispersed in an alcohol is mixed with a fine PTFE powder and the solvent is irradiated with ultrasonic, then the fine powder is disaggregated and mixed with the carbon black to give a satisfactory reaction layer structure. The invention has thus been completed.
On the other hand, since a PTFE dispersion is an aqueous dispersion, addition thereof to a water-immiscible solvent, e.g., solvent naphtha, results in separation into two layers. Consequently, a carbon black does not mix with the PTFE in solvent naphtha.
It was, however, found that when the system composed of the oily phase and the aqueous phase separated therefrom is vigorously mixed, then the fine PTFE particles contained in the PTFE dispersion as the aqueous phase move to the oily phase because these particles in themselves are originally hydrophobic, and the carbon black and the PTFE self-organize in the oily phase.
Namely, it was found that when solvent naphtha containing a hydrophilic carbon black and a hydrophobic carbon black both dispersed therein is vigorously shaken together with a PTFE dispersion, then the fine PTFE particles move from the aqueous phase to the oily phase and self-organize together with the carbon blacks in the solvent naphtha to thereby form a satisfactory reaction layer structure. The invention has thus been completed.
It was furthermore found that according to this means, a gas feed layer structure can be similarly formed when solvent naphtha containing a hydrophobic carbon black dispersed therein is vigorously shaken together with a PTFE dispersion.
Specifically, the above-described objects of the present invention have been achieved by providing the following processes.
(1) A process for producing a reaction layer material or gas feed layer material for a gas diffusion electrode, which comprises the steps of:
dispersing a gas diffusion electrode raw material excluding polytetrafluoroethylene in to an organic solvent to prepare a dispersion;
adding polytetrafluoroethylene to the dispersion to prepare a mixture; and
mixing the mixture.
(2) The process according to the above process (1), wherein polytetrafluoroethylene is added in the form of dispersion.
(3) The process according to the above process (1), wherein polytetrafluoroethylene is added in the form of fine powder.
(4) The process according to the above process (1), wherein the mixing is conducted by or with the aid of ultrasonic irradiation or vigorous agitation or shaking.
(5) The process according to the above process (1), wherein the organic solvent comprises a solvent miscible with water.
(6) The process according to the above process (5), wherein the organic solvent comprises at least one member selected from the group consisting of methanol, ethanol, isopropyl alcohol, ethylene glycol, glycerol, and acetone.
(7) The process according to the above process (1), wherein the organic solvent is a solvent immiscible with water.
(8) The process according to the above process (7), wherein the organic solvent comprises at least one member selected from the group consisting of butyl acetate, solvent naphtha, petroleum naphtha, and fluorochemical solvents.
The process of the invention in which a fine PTFE powder is used comprises considerably simplified steps as compared to a conventional process because a surfactant is not added therein. In the case where solvent naphtha is employed as the solvent which case is shown as Example 10 below, a comparison between the process of the invention and the conventional process is summarized as follows.
The novel process according to the invention roughly comprises the following seven steps.
1. Mixing of a carbon black with solvent naphtha and dispersion of the carbon black
2. Addition of a fine PTFE powder
3. Irradiation with ultrasonic
4. Removal of excess solvent naphtha by compression
5. Sheet formation by rolling
6. Drying and solvent removal
7. Pressing
In contrast, a conventional process in which a surfactant is added in a large amount roughly comprises the following eleven steps.
1. Mixing of a carbon black with water and a surfactant, and dispersion of the carbon black
2. Addition of a PTFE dispersion
3. Addition of an alcohol and mixing
4. Filtration
5. Drying
6. Formation of a curdy mixture by addition of solvent naphtha
7. Sheet formation by rolling
8. Drying
9. Removal of the surfactant
10. Drying
11. Pressing
In preparing the reaction layer material for a gas diffusion electrode, a dispersion is first prepared by mixing a hydrophilic carbon black and a hydrophobic carbon black with an organic solvent. The- amount of the organic solvent is from 9 to 50 parts (the term xe2x80x9cpartsxe2x80x9d used in the specification means xe2x80x9cparts by weightxe2x80x9d unless otherwise indicated) per parts of the carbon blacks. As the carbon blacks are mixed with the solvent in a proportion within that range, the mixture changes from a dry powdery state into a liquid state. After sufficient kneading, an ultrasonic horn is directly applied to the mixture. It is possible to add the carbon blacks little by little to the organic solvent while irradiating the solvent with ultrasonic. Ultrasonic irradiation disperses the carbon blacks to a higher degree and, hence, results in an increased viscosity. Besides ultrasonic irradiation, vigorous agitation or shaking may be used for the mixing.
Subsequently, a necessary amount of a PTFE dispersion is added to the carbon black dispersion and dispersed thereinto by stirring. If desired, ultrasonic irradiation may be conducted. The organic solvent to be used may contain water in such a degree that the carbon blacks can be dispersed into the alcohol. When the PTFE dispersion is used in its undiluted state, a mixture having a curdy consistency results. A liquid mixture can be obtained by using a dilution of the PTFE dispersion with water. A surfactant may be added according to need. After the two liquids have been mixed, the carbon blacks are coexistent with the fine PTFE particles in a mixed state. It is considered that in the case where an alcohol is used as the organic solvent, the alcohol contained in the carbon black dispersion causes the PTFE dispersion to self-organize.
In the case where solvent naphtha was used as the organic solvent, the fine PTFE particles move into the solvent naphtha. Ultrasonic irradiation may be conducted. Use of solvent naphtha in an amount of 10 times results in a mixture having a curdy consistency, while use thereof in an amount of 30 times results in a liquid mixture. The amount of solvent naphtha can hence be regulated according to applications. The PTFE dispersion may be used either in its undiluted state or as a dilution with water.
In the case where a fine PTFE powder is added, in place of a PTFE dispersion, to the carbon black dispersion in an organic solvent, the fine PTFE powder in the powder form is added in a necessary amount and the mixture is lightly stirred to disperse the PTFE particles. In this case, the fine PTFE powder is used in an amount necessary for forming a reaction layer. The fine PTFE powder used here is generally one having an average particle diameter of from 0.2 to 0.3 xcexcm. After the fine PTFE powder has been sufficiently wetted by the organic solvent, e.g., an alcohol, the mixture is irradiated with ultrasonic to disperse the PTFE particles. A surfactant may be added in a small amount according to need; this improves the dispersed state.
Use of the alcohol as an organic solvent in too small an amount results in a mixture having a curdy consistency, while use thereof in too large an amount results in a liquid mixture. Consequently, a use amount thereof is selected while taking account of later treatments.
The dispersion of carbon blacks and fine PTFE particles is applied, as it is as a coating fluid, on a gas feed layer to form a reaction layer. Alternatively, this coating fluid as it is can be applied by spraying to form a reaction layer.
Furthermore, the dispersion can be dried by the spray drying method to obtain a reaction layer material powder. It is possible to prepare the dispersion so that it has a regulated organic-solvent content and to directly form this dispersion into a sheet by the rolling method. It is also possible to use a method comprising drying the dispersion to obtain a reaction layer material powder, adding solvent naphtha to the powder, and forming the resultant mixture into a sheet by the rolling method.
Usable examples of the dispersion medium for dispersing carbon blacks thereinto include alcohols, such as ethanol, methanol and isopropyl alcohol, and acetone. Also usable are organic solvents such as butyl acetate, solvent naphtha, benzene, toluene, xylene, petroleum ether, and fluorine-containing solvents.
In the invention, ingredients such as carbon blacks and PTFE dispersions are referred to as gas diffusion electrode materials, while mixtures obtained by mixing a carbon black dispersion with a PTFE dispersion are referred to as reaction layer raw materials or gas feed layer raw materials.
The gas feed layer material can be produced in the same manner as for the reaction layer material. In the case of a gas feed layer, a hydrophobic carbon black is used as the only carbon black. Consequently, in preparing a dispersion of a hydrophobic carbon black, the carbon black is mixed with an organic solvent, e.g., at least one member selected from the group consisting of methanol, ethanol, isopropyl alcohol, ethylene glycol, glycerol and acetone, or at least one member selected from the group consisting of butyl acetate, solvent naphtha, petroleum naphtha, and fluorochemical solvents. In this case also, the amount of the solvent is from 9 to 50 parts per part by weight of the carbon black. The subsequent procedure also may be the same as for the reaction layer material. Namely, the carbon black is dispersed with ultrasonic irradiation, and a fine PTFE powder in the powder form is added to the resultant dispersion in a necessary amount. This mixture is lightly stirred to disperse the PTFE particles. After the fine PTFE powder has been sufficiently wetted by the solvent, the mixture is irradiated with ultrasonic to disperse the PTFE powder.
Usable examples of the dispersion medium for the carbon black include alcohols such as ethanol, methanol, and isopropyl alcohol, ketones such as acetone, esters such as acetic esters, and hydrocarbon solvents such as solvent naphtha and petroleum ether. Fluorochemical solvents also can be used satisfactorily.
In the drying of the reaction layer material which has been applied or in the spray drying of the dispersion for a reaction layer, the organic solvent vaporizes. By collecting and recovering this organic solvent vapor, not only the cost of the production of the reaction layer material or the like can be reduced, but also problems which may be aroused when the organic solvent vapor is released can be avoided.